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| United States Patent |
6,091,214
|
|
Yamawaki
, et al.
|
July 18, 2000
|
Electric power steering apparatus
Abstract
An electric power steering apparatus comprises a control unit for
drive-controlling a motor to apply an assist torque to a steering system
based on signals from various sensors. The control unit comprises a slip
angle difference predicting section for predicting a difference between a
slip angle of front wheels and a slip angle of rear wheels, and a
correcting section for correcting, based on an angle difference signal
outputted from the slip angle difference predicting section, a target
torque signal supplied to the motor. Behavior of a vehicle is predicted
from the angle difference signal and the target torque signal is corrected
by a correction amount corresponding to the angle difference signal so
that the motor can be drives controlled with the influence of a change in
a road surface reaction force taken into consideration.
| Inventors:
|
Yamawaki; Shigeru (Wako, JP);
Shimizu; Yasuo (Wako, JP);
Takimoto; Shigenori (Wako, JP)
|
| Assignee:
|
Honda Giken Kogyo Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
153395 |
| Filed:
|
September 15, 1998 |
Foreign Application Priority Data
| Sep 16, 1997[JP] | 9-251199 |
| Sep 03, 1998[JP] | 10-249730 |
| Current U.S. Class: |
318/52; 180/443; 318/433; 318/587 |
| Intern'l Class: |
B61C 015/08; B62D 005/04 |
| Field of Search: |
318/432-434,52,85,585-587
180/443-446
|
References Cited
U.S. Patent Documents
| 3687242 | Aug., 1972 | Green | 188/181.
|
| 3707312 | Dec., 1972 | Drutchas et al. | 303/116.
|
| 4751978 | Jun., 1988 | Drutchas et al. | 180/446.
|
| 4830127 | May., 1989 | Ito et al. | 180/446.
|
| 5116254 | May., 1992 | Sano et al. | 180/412.
|
| 5473231 | Dec., 1995 | McLaughlin et al. | 318/433.
|
| 5475289 | Dec., 1995 | McLaughlin et al. | 318/432.
|
| 5504403 | Apr., 1996 | McLaughlin | 318/432.
|
| 5762157 | Jun., 1998 | Uehara | 180/197.
|
| Foreign Patent Documents |
| 5-58318 | Mar., 1993 | JP.
| |
Primary Examiner: Sircus; Brian
Attorney, Agent or Firm: Merchant & Gould P.C.
Claims
What is claimed is:
1. An electric power steering apparatus comprising:
a steering torque sensor for detecting a steering torque of a steering
system;
an electric motor for applying an assist torque to said steering system;
and
a control unit having a target torque signal setting section for setting a
target torque signal based on a steering torque signal from said steering
torque sensor.
wherein said control unit comprises a slip angle difference predicting
section for predicting a difference between a slip angle of front wheels
and a slip angle of rear wheels, and a correcting section for correcting
the target torque signal based on an angle difference signal from said
slip angle difference predicting section.
2. An electric power steering apparatus according to claim 1, further
comprising a turn angle sensor for detecting a turn angle of said front
wheels, a vehicle speed sensor for detecting a speed of a vehicle carrying
said apparatus, and a yaw rate sensor for detecting a yaw rate produced in
said vehicle, wherein said slip angle difference predicting section is
designed to calculate the angle difference based on a turn angle signal
from said turn angle sensor, a vehicle speed signal from said vehicle
speed sensor, a yaw rate signal from said yaw rate sensor, and dimensional
parameters of said vehicle.
3. An electric power steering apparatus according to claim 1, wherein said
correcting section further comprises;
an understeering correction amount outputting section for outputting an
understeering correction amount;
an oversteering correction amount outputting section for outputting an
oversteering correction amount;
a first direction determining section for determining coincidence or
noncoincidence between a direction of the angle difference signal detected
by said slip angle difference predicting section and a direction of the
yaw rate signal from said yaw rate sensor; and
a selecting section for selecting said oversteering correction amount
outputting section when a determining signal from said first direction
determining section indicates coincidence of the directions and selecting
said understeering correction amount outputting section when the
determining signal indicates noncoincidence of the directions.
4. An electric power steering apparatus according to claim 3, wherein said
correction section further comprises:
a subtracting correction section for subtraction-correcting the target
torque signal with a subtracting correction signal corresponding to the
understeering correction amount from said understeering correction amount
outputting section; and
a subtracting correction section for subtraction-correcting the target
torque signal with a subtracting correction signal corresponding to the
oversteering correction amount from said oversteering correction amount
outputting section.
5. An electric power steering apparatus according to claim 3, wherein said
correcting section further comprises;
an angle difference change amount calculating section for calculating an
amount of change of the angle difference signal; and
an angle difference change coefficient generating section for outputting an
angle difference change coefficient corresponding to an angle difference
change signal from said angle difference change amount calculating
section, the angle difference change coefficient being used to correct the
understeering correction amount and the oversteering correction amount.
6. An electric power steering apparatus according to claim 1, wherein said
correcting section further comprises:
an understeering correction amount outputting section for outputting an
understeering correction amount; an oversteering correction amount
outputting section for outputting an oversteering correction amount;
a countersteering correction amount outputting section for outputting a
countersteering correction amount;
a first direction determining section for determining coincidence or
noncoincidence between a direction of the angle difference signal from
said slip angle difference predicting section and a direction of a yaw
rate signal from a yaw rate sensor provided in said apparatus for
detecting a yaw rate produced in said vehicle;
a second direction determining section for determining coincidence or
noncoincidence between the direction of the angle difference signal from
said slip angle difference predicting section and a direction of the
steering toque signal from said steering torque sensor; and
a selecting section for selecting said oversteering correction amount
outputting section when results of determination by said first direction
determining section indicate coincidence of the directions, selecting said
understeering correction amount outputting section when both results of
determination by said first direction determining section and the second
direction determining section indicate noncoincidence of the directions
and selecting said countersteering correction amount outputting section
when a result of determination by said first direction determining section
indicates noncoincidence of the directions and a result of determination
of said second direction determining section indicates coincidence of the
directions.
7. An electric power steering apparatus according to claim 6, wherein said
correcting section further comprises:
a subtracting correction section for subtraction-correcting the target
torque signal with a subtracting correction signal corresponding to the
understeering correction amount from said understeering correction amount
outputting section;
a third direction determining section for determining coincidence or
noncoincidence between a direction of a differentiated value of the angle
difference signal and a direction of the steering torque signal; and
an adding-subtracting correction section for addition-correcting the target
torque signal with an adding correction signal corresponding to the
oversteering correction amount from said oversteering correction amount
outputting section or to the countersteering correction amount from said
countersteering correction amount outputting section when a result of
determination of said third direction determining section indicates
non-coincidence of the directions and for subtraction-correcting the
target torque signal with a subtracting correction signal corresponing to
the oversteering correction amount from said oversteering correction
amount outputting section or to the countersteering correction amount from
said countersteering correction amount outputting section when the
determination result from said third direction determining section
indicates coincidence of the directions.
8. An electric power steering apparatus according to claim 6, wherein said
correcting section further comprises:
an angle difference change amount calculating section for calculating an
amount of change of the angle difference signal; and
is an angle difference change coefficient generating section for outputting
an angle difference change coefficient corresponding to an angle
difference change signal from said angle difference change amount
calculating section, the angle difference change coefficient being used to
correct the understeering correction amount, the oversteering correction
amount and the countersteering correction amount.
9. An electric power steering apparatus according to claim 4, wherein said
correcting section further comprises:
an angle difference change amount calculating section for calculating an
amount of change of the angle difference signal; and
an angle difference change coefficient generating section for outputting an
angle difference change coefficient corresponding to an angle difference
change signal from said angle difference change amount calculating
section, the angle difference change coefficient being used to correct the
understeering correction amount and the oversteering correction amount.
10. An electric power steering apparatus according to claim 7, wherein said
correcting section further comprises:
an angle difference change amount calculating section for calculating an
amount of change of the angle difference signal; and
an angle difference change coefficient generating section for outputting an
angle difference change coefficient corresponding to an angle difference
change signal from said angle difference change amount calculating
section, the angle difference change coefficient being used to correct the
understeering correction amount, the oversteering correction amount and
the countersteering correction amount.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an electric power steering apparatus
for reducing a steering force of a driver by directly applying power of an
electric motor to a steering system and, more particularly, to an electric
power steering apparatus capable of informing a drive of a change in a
road reaction force to thereby make the driver carry out proper steering.
2. Description of the Related Art
There has been known an electric power steering apparatus which comprises a
control unit, a motor driver and a motor. In the apparatus, a target
torque signal for driving the motor is generated by the control unit in
correspondence with a steering torque generated by turning a steering
wheel. A motor control signal for drive-controlling the motor based on the
target torque signal is supplied to the motor driver composed of a bridge
circuit; The motor is then PWM-driven via the motor driver to thereby
apply an assist torque produced by the motor to a steering system.
The control unit drive-controls the motor by causing a signal corresponding
to a motor current to be fed back (negative feedback) to the target torque
signal to quickly equalize the motor current flowing through the motor.
Further, the target torque signal is corrected by a vehicle speed signal
detected by a vehicle speed sensor, the target torque signal is reduced
with an increase in the vehicle speed, a sufficiently large assist torque
is added to the steering system when the vehicle speed is low and when the
vehicle speed is high, a small assist torque is added to the steering
system, thereby achieving reduction in the steering force of a driver when
the vehicle speed is low and behavioral stability of the vehicle when the
vehicle speed is high.
Further, there has been disclosed a conventional electric power steering
apparatus in Japanese Patent Laid-Open Publication No. HEI-5-58318 in
which when skidding of a vehicle is considerable, road information is
transmitted to a driver via a steering wheel by magnifying a road reaction
force from a road surface by reducing an assist torque relative to a
steering torque.
In the electric power steering apparatus disclosed in Japanese Patent
Laid-Open Publication No. HEI-5-58318, there are provided a vehicle speed
sensor, a steering angle sensor and a lateral acceleration sensor. In the
apparatus, a reference lateral acceleration G0 which is assumed to arise
in a vehicle when the latter undergoes no disturbance is determined from a
vehicle speed V detected by the vehicle speed sensor and a steering angle
.theta. detected by the steering angle sensor. Determination is then made
as to whether an absolute value .vertline.Gact-G0.vertline. which is a
difference between an actual lateral acceleration Gact detected, as
actually applied to the vehicle, by the lateral acceleration sensor and a
reference lateral acceleration G0, is larger than a predetermined value
"g". When it is larger (.vertline.Gact-G0.vertline.>g), skidding (lateral
slipping) of the vehicle is regarded to be large and an assist
characteristic map for low road surface friction coefficient .mu. road
which is previously set is selected. When it is small
(.vertline.Gact-G0.vertline.<g), skidding of the vehicle is regarded to be
small, an assist characteristic map for high .mu. road set in advance is
selected, and an assist amount is controlled in accordance with a steering
force.
In the disclosed electric power steering apparatus, there is a tendency
that the driver cannot feel accurately the behavior of the vehicle via the
steering wheel as information since a subtle change in a road reaction
force caused by the behavior of the vehicle is restrained owing to its
arrangement in which an assist torque corresponding to the steering force
of the driver is added to the steering system.
In recent years, such a tendency has been made significant by a tendency of
reduction in a road reaction force resulting from reduction in the
steering force and a tendency of making a steering gear ratio as small as
possible.
With regard to such a steering feeling in respect of the low road reaction
force, it is desired of a driver to carry out accurate steering operation
for the behavior of the vehicle by accurately feeling the road reaction
force in a critical region of the behavior of the vehicle or in operation
at emergency.
For example, when a vehicle is likely to spin in a critical region of the
behavior of the vehicle, a driver needs to swiftly carry out optimum
steering operation by grasping the behavior of the vehicle.
It is the most readily available and effective method to become conscious
of a change in a road reaction force associated with the behavior of the
vehicle for a driver to grasp the vehicle behavior.
In the electric power steering apparatus disclosed in Japanese Patent
Laid-Open Publication No. HEI-5-58318, the assist characteristic map for
high .mu. road or low .mu. road is selected based on a difference between
the actual, lateral acceleration Gact actually applied on the vehicle and
the reference lateral acceleration G0 where the vehicle undergoes no
disturbance. However, the reference lateral acceleration G0 is difficult
to set since it varies with a road surface friction coefficient .mu..
Although a friction coefficient (.mu.) sensor may be mounted on a vehicle
to set the reference lateral acceleration G0 in correspondence with the
road friction coefficient .mu., the friction coefficient (.mu.) is
difficult to detect accurately even when the friction coefficient (.mu.)
sensor is used.
For example, the lateral acceleration G with a parameter of the friction
coefficient (.mu.) in respect of a steering .theta. is provided with a
linear characteristic when the steering angle .theta. falls in a
predetermined range. However, when the steering angle .theta. exceeds the
predetermined range, the characteristic becomes a nonlinear and the
reference lateral acceleration G0 cannot be set.
Further, in the conventional electric power steering apparatus disclosed in
JP Pat. Laid-Open Publication No. JP-HEI-58318, skidding of a vehicle is
determined based on the absolute value .vertline.Gact-G0.vertline. of the
difference between the actual lateral acceleration Gact and the reference
lateral acceleration G0 and accordingly, although whether skidding of the
vehicle is large or small can be determined, whether skidding, of the
vehicle is caused by oversteering or understeering cannot be determined
and the behavior of the vehicle cannot be accurately informed to a driver
as the reaction force via a steering wheel.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an electric
power steering apparatus capable of detecting an oversteering state or an
understeering state of behavior of a vehicle in correspondence with a road
state, transmitting road information (reaction force) to a driver by
applying a auxiliary steering force corresponding to the oversteering
state or the understeering state to a steering system, and having the
driver carry out an optimum steering operation in accordance with an
intention of the driver.
In order to resolve the above-described problems, according to an aspect of
the present invention, there is provided an electric power steering
apparatus having a control unit which comprises a slip angle difference
predicting section for predicting a difference between a front wheel slip
angle and a rear wheel slip angle, and a correcting section for correcting
a target torque signal based on an angle difference signal from the slip
angle difference predicting section.
As explained above, the control unit is provided with the slip angle
difference predicting section for predicting the difference between the
front wheel slip angle and the rear wheel slip angle, and the correcting
section for correcting the target torque signal based on the angle
difference signal from the slip angle difference predicting section. As a
result, the behavior of the vehicle can be predicted from the angle
difference signal while the target torque signal can be corrected by a
correction amount corresponding to the angle difference signal.
Accordingly, the motor for applying the assist torque to the steering
system can be controlled in consideration of influence of a change in the
road reaction.
The slip angle difference predicting section calculates the angle
difference based on a turn angle signal detected by a turn angle sensor
for detecting a turn angle of the front wheel, a vehicle speed signal
detected by a vehicle speed sensor, a yaw rate signal detected by a yaw
rate sensor, and dimensional parameters of a vehicle.
In the slip angle difference predicting section, the angle difference is
calculated based on the turn angle signal detected by the turn angle
sensor for detecting the turn angle of the front wheel, the vehicular
speed signal detected by the vehicular speed sensor, the yaw rate signal
detected by the yaw rate sensor and dimensional parameters of the vehicle.
Consequently, the angle difference can be calculated by using the existing
sensors mounted on the vehicle without using an additional sensor for
actually detecting the angle difference.
In a desired form, the correcting section comprises an understeering
correction amount outputting section for outputting an understeering
correction amount, an oversteering correction amount outputting section
for outputting an oversteering correction amount, a first direction
determining section for determining coincidence or noncoincidence between
a direction of the angle difference signal detected by the slip angle
difference predicting section and a direction of the yaw rate signal, and
a selecting section for selecting the oversteering correction amount
outputting section when a determining signal from the first direction
determining section indicates coincidence of the directions and selecting
the understeering correction amount outputting section when the
determining signal indicates noncoincidence of the directions.
As set forth above, the correcting section comprises the understeering
correction amount outputting section for outputting the understeering
correction amount, the oversteering correction amount outputting section
for outputting the oversteering correction amount, the first direction
determining section for determining coincidence or noncoincidence between
the direction of the angle difference signal detected by the slip angle
difference predicting section and the direction of the yaw rate signal,
and the selecting section for selecting the oversteering correction amount
outputting section when the determining signal from the first direction
determining section indicates coincidence of the directions and selecting
the understeering correction amount outputting section when the
determining signal indicates noncoincidence of the directions. As a
result, whether the behavior of the vehicle is in the oversteering region
or the understeering region can be determined by the direction detected by
the determining signal, whether the behavior of the vehicle is in the
oversteering region or the understeering region is transmitted to the
driver as the reaction force via the steering wheel and the assist torque
applied to the steering system can be corrected in correspondence with the
behavior of the vehicle.
Desirably, the correction section also comprise a subtracting correction
section for subtraction-correcting the target torque signal with a
subtracting correction signal corresponding to an understeering correction
amount from the understeering correction amount outputting section, and a
subtracting correction section for subtraction-correcting the target
torque signal with a subtracting correction signal corresponding to an
oversteering correction amount from the oversteering correction amount
outputting section.
As mentioned above, the correction section comprises the subtracting
correction section for subtraction-correcting the target torque signal
with the subtracting correction signal in correspondence with the
understeering correction amount from the understeering correction amount
outputting section, and the subtracting correction section for
subtraction-correcting the target torque signal with the subtracting
correction signal in correspondence with the oversteering correction
amount from the oversteering correction amount outputting section. As a
result, in the understeering region, a large reaction force can be
transmitted to the driver via the steering wheel by reducing the assist
torque through correction to subtract the understeering correction amount
from the target torque signal while in the oversteering region, a large
reaction force can be transmitted to the driver via the steering wheel by
correction to subtract the oversteering correction amount from the target
torque signal. Further, optimum correction in correspondence with the
oversteering state and under-steering state can be carried out by setting
the oversteering correction amount and the understeering correction amount
independently from each other.
Preferably, the correcting section also comprises an angle difference
change amount calculating section for calculating a change amount of the
angle difference signal, and an angle difference change coefficient
generating section for outputting an angle difference change coefficient
in correspondence with the angle difference change signal from the angle
difference change amount calculating section so that the understeering
correction amount and the oversteering correction amount can be corrected
by the angle difference change coefficient.
An explained above, the correcting section comprises the angle difference
change amount calculating section for calculating the change amount of the
angle difference signal, and the angle difference change coefficient
generating section for outputting the angle difference change coefficient
in correspondence with the angle difference change signal from the angle
difference change amount calculating section so that the understeering
correction amount and the oversteering correction amount can be corrected
by the angle difference change coefficient. As a result, when the behavior
of the vehicle under the understeering state or the oversteering state is
changed rapidly, the behavior of the vehicle, that is, a change in the
reaction force, can be transmitted to the driver via the steering wheel.
In a further specific form of the invention, the correcting section
comprises an understeering correction amount outputting section for
outputting an understeering correction amount, an oversteering correction
amount outputting section for outputting an oversteering correction
amount, a countersteering correction amount outputting section for
outputting a countersteering correction amount, a first direction
determining section for determining coincidence or noncoincidence between
a direction of the angle difference signal detected by the slip angle
difference predicting section and a direction of the yaw rate signal
detected by the yaw rate sensor, a second direction determining section
for determining coincidence or noncoincidence between the direction of the
angle difference signal detected by the slip angle difference predicting
section and a direction of the steering toque signal detected by the
steering torque sensor, and a selecting section for selecting said
oversteering correction amount outputting section when results of
determination by said first direction determining section indicate
coincidence of the directions, selecting the understeering correction
amount outputting section when both determination results by the first
direction determining section and the second direction determining section
indicate noncoincidence of the directions and selecting the
countersteering correction amount outputting section when a determination
result by the first direction determining section indicates noncoincidence
of the directions and a determination result of the second direction
determining section indicates coincidence of the directions.
As discussed above, the correcting section comprises the understeering
correction amount outputting section for outputting the understeering
correction amount, the oversteering correction amount outputting section
for outputting the oversteering correction amount, the countersteering
correction amount outputting section for outputting the countersteering
correction amount, the first direction determining section for determining
coincidence or noncoincidence between the direction of the angle
difference signal detected by the slip angle difference predicting section
and the direction of the yaw rate signal detected by the yaw rate sensor,
the second direction determining section for determining coincidence or
noncoincidence between the direction of the angle difference signal
detected by the slip angle difference predicting section and the direction
of the steering torque signal detected by the steering torque sensor, and
the selecting section for selecting siad oversteering correction amount
outputting section when results of determination by said first direction
determining section indicate coincidence of the directions selecting the
understeering correction amount outputting section when both results of
determination by the first direction determining section and the second
direction determining section indicate noncoincidence of the directions
and selecting the counter-steering correction amount outputting section
when a result of determination by the first direction determining section
indicates noncoincidence of the directions and a result of determination
of the second direction determining section indicates coincidence of the
directions. As a result, whether the behavior of the vehicle is in the
understeering state or the excessive countersteering state is determined
by determining the directions of the angle difference signal, the yaw rate
signal and the steering torque signal and a correction amount in
correspondence with the understeering state or the excessive
countersteering state can be outputted.
In a still further specific form of the invention, the correcting section
comprises a subtracting correction section for subtraction-correcting the
target torque signal with the subtracting correction signal corresponding
to the understeering correction amount from the understeering correction
amount outputting section, a third direction determining section for
determining coincidence or noncoincidence between a direction of a
differentiated value of the angle difference signal and a direction of the
steering torque signal, and an adding-subtracting correction section for
addition-correcting the target torque signal with an adding correction
signal in correspondence with the oversteering correction amount or the
countersteering correction amount from the oversteering correction amount
outputting section or the countersteering correction amount outputting
section when a determination result of the third direction determining
section indicates noncoincidence of the directions and
subtraction-correcting the target torque signal with a subtracting
correction signal in correspondence with the oversteering correction
amount or the countersteering correction amount from the oversteering
correction amount outputting section or the countersteering correction
amount outputting section when the determination result from the third
direction determining section indicates coincidence of the directions.
As stated above, the correcting section comprises the subtracting
correction section for subtraction-correcting the target torque signal
with the subtracting correction signal in correspondence with the
understeering correction amount from the understeering correction amount
outputting section, the third direction determining section for
determining coincidence or noncoincidence between the direction of the
differentiated value of the angle difference signal and the direction of
the steering torque signal, and the adding-subtracting correction section
for addition-correcting the target torque signal with the adding
correction signal in correspondence with the oversteering correction
amount or the countersteering correction amount from the oversteering
correction amount outputting section or the countersteering correction
amount outputting section when the determination result of the third
direction determining section indicates noncoincidence of the directions
and for subtraction-correcting the target torque signal with the
subtracting correction signal in correspondence with the oversteering
correction amount or the countersteering correction amount from the
oversteering correction amount outputting section or the countersteering
correction amount outputting section when the determination result from
the third direction determining section indicates coincidence of the
directions. As a result, by determining the direction of the
differentiated value of the angle difference signal and the direction of
the steering torque signal, whether the countersteering correction amount
is excessively large or excessively small can be transmitted to the driver
through the reaction force by adding the oversteering correction or the
countersteering correction amount to the target torque signal or
subtracting the oversteering correction amount or the countersteering
correction amount from the target torque signal.
In a still further specific form, the correcting section comprises an angle
difference change amount calculating section for calculating a change
amount of the angle difference signal, and an angle difference change
coefficient generating section for outputting an angle difference change
coefficient in correspondence with an angle difference change signal from
the angle difference change amount calculating section so that the
understeering correction amount, the oversteering correction amount and
the countersteering correction amount can be corrected by the angle
difference change coefficient.
As mentioned above, the correcting section comprises the angle difference
change amount calculating section for calculating the change amount of the
angle difference signal, and the angle difference change coefficient
generating section for outputting the angle difference change coefficient
in correspondence with the angle difference change signal from the angle
difference change amount calculating section so that the understeering
correction amount, the oversteering correction amount and the
countersteering correction amount can be corrected by the angle difference
change coefficient. As a result, even when the behavior of the vehicle in
the understeering state, the oversteering state or the countersteering
state is rapidly changed, the change in the behavior of the vehicle can be
quickly transmitted to the driver as a change in the reaction force via
the steering wheel.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will be described
hereinbelow, by way of example only, with reference to the accompanying
drawings, in which:
FIG. 1 is a schematic view illustrating the overall arrangement of an
electric power steering apparatus according to the present invention;
FIG. 2 is a block diagram showing a part of the electric power steering
apparatus according to the present invention;
FIG. 3 is a block diagram showing a part of a control unit and a correcting
section according to one embodiment of the present invention;
FIG. 4 is a block diagram showing a part of an altered form of the
correcting section;
FIG. 5 is a flowchart showing operation of the correcting section of FIG.
4;
FIG. 6 is a Lissajous figure of an actual steering angle .theta. versus a
front and rear wheel slip angle difference .beta.fr of a vehicle carrying
the electric power steering apparatus with the correcting section of FIG.
4;
FIG. 7 is a schematic view illustrating drift running of a vehicle carrying
the electric power steering apparatus according to the present invention;
FIG. 8 is a graph illustrating characteristics of a steering torque signal
T versus a target torque signal IMS;
FIG. 9 is a graph illustrating characteristics of a vehicle speed signal V
versus a vehicle speed coefficient KT;
FIG. 10 is a graph illustrating characteristics of the vehicle speed signal
V versus a vehicle speed coefficient KR;
FIG. 11 is a graph illustrating characteristics of an angle difference
signal .beta.fr versus an understeering correction amount DA;
FIG. 12 is a graph illustrating characteristics of the angle difference
signal .beta.fr versus an oversteering correction amount DO;
FIG. 13 is a graph illustrating characteristics of the angle difference
signal .beta.fr versus a countersteering correction amount DC; and
FIG. 14 is a graph illustrating characteristics of an angle difference
change amount DV versus an angle difference change coefficient KV.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description is merely exemplary in nature and in no way
intended to limit the invention or its application or uses.
Generally, the present invention is directed to an arrangement wherein a
reaction force corresponding to the behavior of a vehicle in an
understeering state, an oversteering state and an excessive
countersteering state is accurately transmitted to a driver through a
steering wheel to thereby assist in steering operation desired by the
driver in accordance with the respective behavior of the vehicle.
Referring to FIG. 1, an electric power steering apparatus 1 comprises a
steering wheel 2, a steering shaft 3, a hypoid gear 4, a rack-and-pinion
mechanism 5 having a pinion 5a and a rack shaft 5b. It also comprises a
tie rod 6, front steered wheels 7 (only one shown), a motor 8 for
providing an assist torque to a steering system, a control unit 13, a
motor driver 14 and a motor current detector 15.
Further, the electric power steering apparatus 1 is provided with a yaw
rate sensor 9 for detecting a yaw rate produced in a vehicle and
outputting a yaw rate signal Y converted into an electric signal
corresponding to the yaw rate, a turn angle sensor 10 for detecting a turn
angle of the front wheel and outputting a turn angle signal .delta.
converted into an electric signal corresponding to the turn angle of the
front wheel, a vehicle speed sensor 11 for detecting a vehicle speed and
outputting a vehicle speed signal V converted into an electric signal
corresponding to the vehicle speed, and a steering torque sensor 12 for
detecting a steering torque produced by the steering wheel 2 and
outputting a steering torque signal T converted into an electric signal
corresponding to the steering torque.
The turn angle signal .delta. may be calculated from a steering angle of
the steering shaft.
Each of the yaw rate signal Y, the turn angle signal .delta. and the
steering torque signal T has a magnitude and a direction and is supplied
to the control unit 13.
In respect of the directions of the yaw rate signal Y, the turn angle
signal .delta., the vehicle speed V and the steering torque signal T, the
clockwise direction is determined to be positive (plus) while the
counterclockwise direction is determined to be negative (minus).
When the steering wheel 2 is operated, a manual steering torque produced in
the steering shaft 3 changes an operational direction of the front wheels
7 via the tie rod 6 by converting a rotational force of the pinion 5a into
a linear movement of the rack shaft 5b in the axial direction via the
rack-and-pinion mechanism 5.
As the motor 8 is driven in accordance with the steering torque signal T
for assisting in the manual steering torque, the torque of the motor is
converted into an assist torque magnified via the hypoid gear 4 and is
applied to the steering shaft 3, whereby the steering force of a driver is
reduced.
The control unit 13 is comprised basically of a microprocessor and includes
various operating units, processing units, determining sections, switching
sections, signal generating section, memories and so forth. The control
unit 13 generates a target torque signal (IMS) corresponding to the
steering torque signal T, a motor control signal V0 (e.g., mixed signal of
an ON signal, an OFF signal and a PWM signal) corresponding to a
difference between the target torque signal (IMS) and a motor torque
signal IMF corresponding to a motor current IM detected by the motor
current detector 15 (negative feedback) and controls the drive of the
motor driver 14 such that the difference is swiftly nullified (to become
0).
Also, the control unit 13 has a front and rear wheel slip angle difference
predicting section and a correcting section, predicts by calculation a
difference between a slip angle of front wheels and a slip angle of rear
wheels (angle difference signal) based on the yaw rate signal Y, the turn
angle signal .delta., the vehicle speed signal V and vehicle dimensional
parameters (wheel base), determines a correction amount in an
understeering region, an oversteering region or an excessive
countersteering region based on the magnitude of the difference (angle
difference signal), and corrects the target torque signal (IMS) by the
correction amount.
Further, the control unit 13 determines that a vehicle state (vehicle
behavior) is in any of the understeering region, the oversteering region
and the excessive countersteering region by comparing a direction (P) of
the difference between the slip angle of the front wheels and the slip of
the rear wheels (angle difference signal), a direction (N) of the yaw rats
signal Y and a direction (S) of the steering torque signal T.
The motor driver 14 comprises a bridge circuit comprising switching
elements of, for example, four power FETs (field effect transistors),
insulated gate bipolar transistors (IGBT) and so on, outputs a PWM
(pulse-width-modulated) motor voltage VM based on the motor control signal
V0, and PWM-drives the motor 8 so that it rotates regularly or reversely.
The motor current detector 15 detects the motor current IM by converting it
into a voltage by a resistor, a hole element or the like connected in
series with the motor 8 and feeds a motor torque signal IMF corresponding
to the motor current IM back to the control unit 13 (negative feedback).
Reference is now made to FIG. 2 showing, in block diagram, part of the
electric power steering apparatus according to the present invention.
As shown in FIG. 2, the control unit 13 comprises a target torque signal
setting section 21, a correcting section 17, a difference calculating
section 22, a drive control section 23 and a slip angle difference
predicting section 16.
The target torque signal setting section 21 has a memory such as a ROM
(Read Only Memory), stores date in correspondence with the torque signal
data T and target torque signal data IMO which have been set in advance
based on experimental values or design values with the vehicle speed V as
a parameter, reads the corresponding target torque signal data IMO based
on the steering torque signal T detected by the steering torque sensor 12
and the vehicle speed signal V detected by the vehicle speed sensor 11,
and supplies the target torque signal IMO to the correcting section 17.
The correcting section 17 has a memory such as a ROM, software-controlled
comparison, switching and calculation functions, stores correction amounts
(understeering correction amount, oversteering correction amount and
countersteering correction amount) in accordance with a slip angle
difference, generates a corresponding correction amount based on an angle
difference signal .beta.fr predicting a difference (.beta.f-.beta.r)
between a front wheel slip angle (.beta.f) and a rear wheel slip angle
(.beta.r) calculated by the slip angle difference predicting section 16,
and supplies a target torque signal IMH produced by correcting the target
torque signal IMO by the correction amount to the difference calculating
section 22.
When the difference between the front wheel slip angle (.beta.f) and the
rear wheel slip angle (.beta.r) is equal to or smaller than a
predetermined value, the vehicle is running normally where the behavior of
the vehicle is stabilized and accordingly, the correction amount from the
correcting section 17 is nullified and the target torque signal IMH
outputted from the correcting section 17 is equal to the target torque
signal IMO (IMH=IMO).
Meanwhile, when the difference between the front wheel slip angle (.beta.f)
and the rear wheel slip angle (.beta.r) exceeds the predetermined value,
the behavior of the vehicle is unstable and therefore, the target torque
signal IMO is corrected by the correction amount from the correcting
section 17 and the target torque signal IMH outputted from the correcting
section 17 is produced by subtracting the correction amount from the
target torque signal IMO (IMH<IMO) or adding the correction amount thereto
(IMH>IMO) in accordance with the behavior of the vehicle.
The difference calculating section 22 is provided with a subtractor or a
subtracting function, calculates a difference .DELTA.I (=IMH-IFM) between
the target torque signal IMH supplied from the correcting section 17 and
the motor torque signal IMF supplied from the motor current detector 15,
and supplies the difference signal .DELTA.I (=IMH-IFM) to the drive
control section 23.
The drive control section 23 has a PID (Proportional plus Integral plus
Derivative) controller and a motor control signal generating section,
subjects the difference signal .DELTA.I supplied from the difference
calculating section 22 to a proportional (P), integral (I) and derivative
(D) control, thereafter generates the PWM motor control signal V0
corresponding to rightward or leftward turning of the steering wheel based
on a mixture signal mixed with signals subjected to the proportional,
integral and derivative (PID) control, and supplies the motor control
signal V0 to the motor driver 14.
The slip angle difference predicting section 16 comprises a memory and a
calculating section, predicts the behavior of the vehicle by calculating
the difference .beta.fr (=.beta.f-.beta.r) between the front wheel slip
angle (.beta.f) and the rear wheel slip angle (.beta.r) based on the
vehicle speed V, the yaw rate signal Y, the turn angle signal .delta. in
correspondence with the turn angle of the front wheels and dimensional
parameters of the vehicle (e.g., wheel base L) previously set in the
memory, and provides the difference .beta.fr to the correcting section 17
as the angle difference signal .beta.fr.
In this way, the control unit 13 can predict the behavior of the vehicle
from the angle difference signal .beta.fr, correct the target torque
signal IMO by the correction amount in correspondence with the angle
difference signal .beta.fr, and drive-control the motor for adding an
assist torque to the steering system with an influence of a change in a
reaction force taken into consideration, since the control unit 13 is
provided with the slip angle difference predicting section 16 for
predicting the difference .beta.fr (=.beta.f-.beta.r) between the front
wheel slip angle (.beta.f) and the rear wheel slip angle (.beta.r), and
the correcting section 17 for correcting the target torque signal IMO
based on the angle difference signal .beta.fr from the slip angle
difference predicting section 16.
Next, reference is made to FIG. 3 which shows, in block diagram, part of
one embodiment of the control unit according to the present invention.
As shown in FIG. 3, the control unit 13 has the target torque signal
setting section 21, a vehicle speed coefficient generating section 18, a
vehicle speed coefficient generating section 19, a multiplying section 24,
the slip angle difference predicting section 16, the correction section 17
and the difference calculating section 22.
The target torque signal setting section 21 pre-stores characteristic data
of the steering torque signal T versus a target torque signal IMS, as
shown in FIG. 8, in its memory such as a ROM. When the steering torque
signal T is supplied from the steering torque sensor 12, the target torque
signal setting section 21 reads a corresponding target torque signals IMS
and provides it to the multiplying section 24.
The vehicle speed coefficient generating section 18 comprises a memory such
as a ROM and pre-stores characteristic data of the vehicle speed V versus
a vehicle speed coefficient KT as shown in FIG. 9. When the vehicle speed
signal V is supplied from the vehicle speed sensor 11, the vehicle speed
coefficient generating section 18 reads a corresponding vehicle speed
coefficient KT and provides it to the multiplying section 24.
With a software-controlled multiplying function, the multiplying section 24
multiplies a target torque signal IMS supplied from the target torque
signal setting section 21 by the vehicle speed coefficient KT supplied
from the vehicle speed coefficient generating section 18 and supplies the
target torque signal IMO in correspondence with the vehicle speed V to a
subtracting section 37 of the correcting section 17.
Since the vehicle speed coefficient KT has a characteristic such that it
decreases with an increase in the vehicle speed signal V, as shown in FIG.
9, the target torque signal IMO shown in FIG. 8 is formed as the target
torque signal IMO which is corrected to decrease with the increase in the
vehicle speed signal V.
The vehicle speed coefficient generating section 19 comprises a memory such
as a ROM and pre-stores characteristic data of the vehicle speed V versus
a vehicle speed coefficient KR, as shown in FIG. 10. When the vehicle
speed signal V is supplied from the vehicle speed sensor 11, the vehicle
speed coefficient generating section 19 reads a corresponding vehicle
speed coefficient KR and provides it to a multiplying section 35 and a
multiplying section 36 of the correcting section 17.
The slip angle difference predicting section 16 has a memory and a
calculating function, calculates the difference .beta.fr
(=.beta.f-.beta.r) between the font wheal slip angle (.beta.f) and the
rear wheel slip angle (.beta.r) from Equation (1) based on the vehicle
speed V, the yaw rate signal Y, the turn angle signal .delta. and the
dimensional parameter L (e.g., wheel base) of the vehicle preset in the
memory and supplies the angle difference signal .beta.fr to a selecting
section 31, a first direction determining section 32 and an angle
difference change amount calculating section 39 of the correcting section
17.
.beta.fr=Y*L/V-.delta. (1)
Each of the front wheal slip angle (.beta.f) and the rear wheel slip angle
(.beta.r) represents an angle in a progressing direction of tires with an
orientation of the tires as a reference and accordingly, when the steering
wheel is turned in a clockwise direction, the progressing direction of the
tires is in a counterclockwise direction relative to the direction of the
front wheel tires and when the clockwise direction is determined to be
positive (plus), the direction of the front wheel slip angle (.beta.f)
becomes negative (minus).
Similarly, the rear wheel slip angle (.beta.r) also becomes negative
(minus) and a direction (sign) of the angle difference signal .beta.fr is
designated as negative (minus) until an absolute value
.vertline..beta.r.vertline. of the rear wheel slip angle (.beta.r) becomes
equal to or larger than an absolute value .vertline..beta.f.vertline. of
the front wheel slip angle (.beta.f)
(.vertline..beta.f.vertline..gtoreq..vertline..beta.f.vertline.).
Further, lateral acceleration G can be substituted for the yaw rate signal
Y supplied to the first direction determining section 32.
In this way, the slip angle difference predicting section 16 calculates the
angle difference .beta.fr based on the turn angle signal .delta. detected
by the turn angle sensor 10 for detecting the turn angle of the front
wheel, the vehicle speed V detected by the vehicle speed sensor 11, the
yaw rate signal Y detected by the yaw rate sensor 9 and the dimensional
parameter L of the vehicle. Accordingly, the angle difference .beta.fr can
be calculated by using the existing sensors mounted on the vehicle without
actually detecting the angle difference.
The correcting section 17 comprises the selecting section 31, the first
direction determining section 32, an understeering correction amount
outputting section 33, an oversteering correction amount outputting
section 34, the multiplying section 35, the multiplying section 36, the
subtracting section 37 as a subtracting correction section, a subtracting
section 41, the angle difference change amount calculating section 39 and
an angle difference change coefficient generating section 40.
The selecting section 31 has a software-controlled switching function,
switches a changeover switch based on a determining signal H0 supplied
from the first direction determining section 32, and supplies the angle
difference signal .beta.fr fed from the slip angle difference predicting
section 16 to the understeering correction amount outputting section 33 or
the oversteering correction amount outputting section 34.
The first direction determining section 32 has a sign comparing function,
supplies the selecting section 31 with the determining signal H0 at, for
example, H level based on a direction signal P of the angle difference
signal .beta.fr supplied from the slip angle difference predicting section
16 and a direction signal N of the yaw rate Y supplied from the yaw rate
sensor 9, when the direction signal P and the direction signal N coincide
with each other (signs are same), and supplies the selecting section 31
with the determining signal H0 at, for example, L level when the direction
signal P and the direction signal N differ from each other (signs are
different).
When the direction signal P of the angle difference signal .beta.fr and the
direction signal N of the yaw rate signal Y differ from each other
(noncoincidence), for example, as the yaw rate Y is directed in the
clockwise direction and the slip angle (.beta.f) of the front wheel in the
counterclockwise direction is larger than the slip angle (.beta.r) of the
rear wheel in the counterclockwise direction, the direction signal N of
the yaw rate signal Y becomes plus (+) and the direction signal P of the
angle difference signal .beta.fr becomes minus (-), the behavior of the
vehicle is determined to be in the understeering region and the selecting
section 31 selects the understeering correction amount outputting section
33.
When the direction signal P of the angle difference signal .beta.fr and the
direction signal N of the yaw rate signal Y are the same (coincide with
each other), in the case where, for example, the yaw rate Y is directed in
the clockwise direction and the slip angle (.beta.r) of the rear wheel in
the counterclockwise direction is larger than the slip angle (.beta.f) of
the front wheel in the counterclockwise direction, the direction signal N
of the yaw rate signal Y becomes plus (+) and the direction signal P of
the angle difference signal .beta.fr becomes plus (+), behavior of the
vehicle is determined to be in the oversteering region and the selecting
section 31 selects the oversteering correction amount outputting section
34.
Strong understeering region of the behavior of the vehicle is a state in
which even when the steering wheel is turned further from a current
steering state, the vehicle is not curved further and is a steering region
for making a driver feel a strong reaction force and informing the driver
that the steering wheel had better be turned back.
In a weak understeering region, correction of a road reaction force is not
needed and accordingly, as shown in FIG. 11, a dead zone region of an
understeering correction amount DA in respect of the angle difference
signal .beta.fr is set to be large.
Meanwhile, a strong oversteering region of the vehicle is a state in which
the vehicle may spin when the vehicle stays as it is and a driver is made
feel a strong reaction force and countersteering is rendered easy to carry
out.
The understeering correction amount outputting section 33 comprises a
memory such as a ROM, pre-stores characteristic data of the absolute value
.vertline..beta.fr.vertline. of the angle difference signal versus the
understeering correction amount DA shown in FIG. 11. When the angle
difference signal .beta.fr is supplied from the selecting section 31, the
understeering correction amount outputting section 33 reads a
corresponding understeering correction amount DA and supplies the
understeering correction amount signal DA to the multiplying section 35.
The oversteering correction amount outputting section 34 comprises a memory
such as a ROM and pre-stores characteristic data of the absolute value
.vertline..beta.fr.vertline. of the angle difference signal versus an
oversteering correction amount DO shown in FIG. 12. When the angle
difference signal .beta.fr is supplied from the selecting section 31, the
oversteering correction amount outputting section 34 reads a corresponding
oversteering correction amount DO and supplies the oversteering correction
amount signal DO to the multiplying section 36.
As explained above, the correcting section 17 has the understeering
correction amount outputting section 33 for outputting the understeering
correction amount DA, the oversteering correction amount outputting
section 34 for outputting the oversteering correction amount DO, the first
direction determining section 32 for determining coincidence or
noncoincidence of the direction P of the angle difference signal .beta.fr
detected by the slip angle difference predicting section 16 and the
direction N of the yaw rate signal Y, and the selecting section 31 for
selecting the understeering correction amount outputting section 33 when
the determining signal HO from the first direction determining section 32
determines that the directions do not coincide with each other, and
selecting the oversteering correction amount outputting section 34 when
the determining signal HO determines that the directions coincide with
each other. Accordingly, whether the behavior of the vehicle is in the
understeering region or the oversteering region can be determined by the
direction detected by the determining signal HO and the target torque
signal IMS can be corrected in correspondence with the behavior of the
vehicle.
For the understeering correction amount and the oversteering correction
amount, respective dead zones are provided as shown in FIGS. 11 and 12,
whereby optimum correction in accordance with the understeering state or
the oversteering state can be carried out.
The multiplying section 35 has a sofrware-controlled multiplying function,
multiplies the vehicle speed coefficient KR, the understeering correction
amount signal DA and an angle difference change coefficient KV, and
supplies an understeering correction amount signal IDA (=KR*KV*DA) as a
subtracting correction signal to the subtracting section 37.
The understeering correction amount signal IDA is produced by correcting
the understeering correction amount DA shown in FIG. 11 by the vehicle
speed coefficient KR shown in FIG. 10. Thus, in a low vehicle speed
region, the understeering correction amount DA is nullified and the
correction is not carried out. For a region from a middle vehicle speed to
a high vehicle speed, the understeering correction amount signal IDA may
be set the same as in the characteristic of the understeering correction
amount DA.
The multiplying section 36 has a software-controlled multiplying function,
multiplies the vehicle speed coefficient KR, the oversteering correction
amount signal DO and the angle difference change coefficient KV, and
supplies the oversteering correction amount signal IDO (=KR*KV*DO) as a
subtracting correction signal to the subtracting section 41.
The oversteering correction amount signal IDO is produced by correcting the
overstesring correction amount DO shown in FIG. 12 by the vehicle speed
coefficient KR shown in FIG. 10. Thus, in a low vehicle speed region, the
oversteering correction amount DO is nullified and the correction is not
carried out. In a region from a middle vehicle speed to a high vehicle
speed, the oversteering correction amount signal IDO may be set to be the
same as in the characteristic of the oversteering correction amount DO.
In comparison with the understeering correction amount DA, in the
oversteering correction amount DO, the dead zone is set to be narrow and
the inclination is set to be small.
The subtracting section 37 has a software-controlled subtracting function,
constitutes a subtracting correction section operated in the understeering
region, calculates the difference (=IMO-IDA) between the target torque
signal IMO supplied from the multiplying section 24 and the understeering
correction amount signal IDA (=KR*KV*DA) which is a subtracting correction
signal supplied from the multiplying section 35, and supplies a new target
torque signal IMA (=IMO-IDA) to the difference calculating section 22 as a
target torque signal IMH via the subtracting section 41.
When the understeering correction amount signal IDA is outputted, the
oversteering correction amount signal IDO is not outputted (IDO=0).
Accordingly, the target torque signal IMH becomes equal to the target
torque signal IMA (IMH=IMA).
The subtracting section 41 has a software-controlled subtracting function,
constitutes a subtracting correction section operated in the oversteering
region, calculates the difference (=IMO-IDO) between the target torque
signal IMO supplied from the multiplying section 24 and the oversteering
correction amount signal IDO (=KR*KV*DO) which is a subtracting correction
signal supplied from the multiplying section 36, and supplies it to the
difference calculating section 22 as the new target torque signal IMH.
As already explained, the correcting section 17 is provided with the
subtracting correction section 37 for subtraction-correcting the target
torque signal IMO with the subtracting correction signal IDA corresponding
to the understeering correction amount DA from the understeering
correction amount outputting section 33 and the subtracting correction
section 41 for subtraction-correcting the target torque signal IMO with
the subtracting correction signal IDO corresponding to the oversteering
correction amount DO from the oversteering correction amount outputting
section 34. Accordingly, a large reaction force can be transmitted to a
driver via the steering wheel by reducing the assist torque by
subtraction-correcting the target torque signal IMO with the subtracting
correction signal IDA in the understeering region, and a large reaction
force can be transmitted thereto by subtraction-correcting the target
torque signal IMO with the subtracting correction signal IDO in the
oversteering region.
Being provided with a differentiating function, the angle difference change
amount calculating section 39 differentiates the angle difference signal
.beta.fr supplied from the slip angle difference predicting section 16 and
supplies an angle difference change amount signal DV (=d .beta.fr/dt) to
the angle difference change coefficient generating section 40.
The angle difference change coefficient generating section 40 comprises a
memory such as a RON which pre-stores characteristic data of the angle
difference change amount DV versus the angle difference change coefficient
KV shown in FIG. 14. When the angle difference change amount signal DV is
supplied, the angle difference change coefficient generating section 40
reads a corresponding angle difference change coefficient KV and supplies
it to the multiplying section 35 and the multiplying section 36.
The angle difference change amount DV represents a change in the angle
difference signal .beta.fr and hence a change in the timewise behavior of
the vehicle and can thus generate the understeering correction amount
signal IDA (=KR*KV*DA) or the oversteering correction amount signal IDO
(=KR*KV*DO) in correspondence with a rapid change in the behavior of the
vehicle.
Because the correcting section 17 has the angle difference change amount
calculating section 39 for calculating the change amount DV of the angle
difference signal .beta.fr and the angle difference change coefficient
generating section 40 for outputting the angle difference change
coefficient KV in correspondence with the angle difference change signal
DV from the angle difference change amount calculating section 39 and
corrects the understeering correction amount DA and the oversteering
correction amount DO by the angle difference change coefficient KV, even
when the behavior of the vehicle is rapidly changed, a rapid change in the
reaction force can be transmitted to the driver via the steering wheel.
Referring to FIG. 4, discussion will be made next as to another embodiment
of the correcting section. Note that the altered correcting section is
designed to determine, in addition to the understeering state and the
oversteering state, an excessive countersteering state and to carry out
corrections in correspondence with respective behaviors of the vehicle.
As shown in FIG. 4, a correcting section 50 includes a selecting section
51, a second direction determining section 52, a countersteering
correction amount outputting section 53, a third direction determining
section 54, a selecting section 55, an adding section 56 and an adding
section 57 in addition to the selecting section 31, the first direction
determining section 32, the understeering correction amount outputting
section 33, the oversteering correction amount outputting section 34, the
multiplying section 35, the multiplying section 36, the subtracting
section 37 and adding section 38 constituting an adding-subtracting
correction section, the angle difference change amount calculating section
39 and the angle difference change coefficient generating section 40.
In relation to FIG. 3, discussion has already been made on the selecting
section 31, the first direction determining section 32, the understeering
correction amount outputting section 33, the oversteering correction
amount outputting section 34, the multiplying section 35, the multiplying
section 36, the subtracting section 37 constituting the subtracting
correction section, the angle difference change amount calculating section
39, and the angle difference change coefficient generating section 40.
Therefore, a further detailed explanation thereof will be omitted and only
their partial elements will be included in the following explanation as
they become necessary.
The selecting section 51 has a software-controlled switching function,
switches a changeover switch based on a determining signal H02 supplied
from the second direction determining section 52, and supplies the angle
difference signal .beta.fr fed from the selecting section 31 to the
understeering correction amount outputting section 33 or the
countersteering correction amount outputting section 53.
The second direction determining section 52 has a sign comparing function,
supplies the determining signal H02 at, for example, H level to the
selecting section 51 based on the direction signal P of the angle
difference signal .beta.fr fed from the slip angle difference predicting
section 16 and a direction signal S of the steering torque signal T fed
from the steering torque sensor 12 when the direction signal P and the
direction signal S coincide with each other (signs are same), and supplies
the determining signal H02 at, for example, L level to the selecting
section 51 when the direction signal P and direction signal S differ from
each other (signs are different).
When the direction signal P of the angle difference signal .beta.fr and the
direction signal S of the steering torque signal T differ from each other
(noncoincidence), for example, where the steering torque signal T is
directed in a clockwise direction and the slip angle (.beta.f) of the
front wheels in a counterclockwise direction is larger than the slip angle
(.beta.r) of the rear wheels in the counterclockwise direction, the
direction signal S of the steering torque signal T becomes a plus (+) and
the direction signal P of the angle difference signal .beta.fr becomes a
minus (-), the behavior of the vehicle is determined to be in the
understeering region and the selecting section 51 selects the
understeering correction amount outputting section 33 (designated by
broken line).
Meanwhile, when the direction signal P of the angle difference signal
.beta.fr and the direction signal S of the steering torque signal T are
the same (coincide), for example, where the steering torque signal T is
directed in the clockwise direction while the slip angle (.beta.r) of the
rear wheels in the counterclockwise direction is larger than the slip
angle (.beta.f) of the front wheels in the counterclockwise direction, the
direction signal S of the steering torque signal T becomes a plus (+) and
the direction signal P of the angle difference signal .beta.fr becomes a
plus (+), the behavior of the vehicle is determined to be in the excessive
countersteering region and the selecting section 51 selects the
countersteering correction amount outputting section 53 (designated by
bold line).
With regard to the first direction determining section 32 and the selecting
section 31, note that where, for example, the yaw rate signal Y is in the
clockwise direction and the front wheel slip angle (.beta.f) in the
counterclockwise direction is larger than the rear wheel slip angle
(.beta.r) in the counterclockwise direction, the direction signal N of the
yaw rate signal Y becomes a plus (+) while the direction signal P of the
angle difference signal .beta.fr becomes a minus (-), whereupon the
selecting section 31 selects the selecting section 51 (designated by
broken line) while the selecting section 51 selects the understeering
correction amount outputting section 33 or the countersteering correction
amount outputting section 53 in accordance with a result of determination
of the second direction determining section 52 mentioned above.
In regard to the first direction determining section 32 and the selecting
section 31, note also that when the direction signal P of the angle
difference signal .beta.fr and the direction signal N of the yaw rate
signal Y are the same (coincidence), for example, where the yaw angular
rate signal Y is in the clockwise direction and the rear wheel slip angle
(.beta.r) in the counterclockwise direction is larger than the front wheel
slip angle (.beta.f) in the counter-clockwise direction, the direction
signal N of the yaw rate signal Y becomes a plus (+) while the direction
signal P of the angle difference signal .beta.fr becomes a plus (+),
whereupon the behavior of the vehicle is determined to be in the
oversteering region and the selecting section 31 selects the oversteering
correction amount outputting section 34 (designated by bold line).
The countersteering correction amount outputting section 53 has a memory
such as a ROM and pre-stores characteristic data of the absolute value
.vertline..beta.fr.vertline. of the angle difference signal versus a
countersteering correction amount DC shown in FIG. 13. When the angle
difference signal data .beta.fr is supplied from the selecting section 51,
the countersteering correction amount outputting section 53 reads out a
corresponding countersteering correction amount DC and supplies a
countersteering correction amount signal DC to the selecting section 55
via the adding section 57.
Being thus arranged, the correcting section 50 determines that the behavior
of the vehicle is in the understeering state and selects the understeering
correction amount outputting section 33 when the first direction
determining section 32 determines that the direction signal P of the angle
difference signal .beta.fr and the direction signal N of the yaw rate
signal Y do not coincide with each other and the second direction
determining section 52 determines that the direction signal P of the angle
difference signal .beta.fr and the direction signal S of the steering
torque signal T do not coincide with each other.
Further, the correcting section 50 determines that the behavior of the
vehicle is in the excessive countersteering state and selects the
countersteering correction amount outputting section 53 when the first
direction determining section 32 determines that the direction signal P of
the angle difference signal .beta.fr and the direction signal N of the yaw
rate signal Y do not coincide with each other and the second direction
determining section 52 determines that the direction signal P of the angle
difference signal .beta.fr and the direction signal S of the steering
torque signal T coincide with each other.
The third direction determining section 54 has a sign comparing function,
supplies a determining signal H03 at, for example, H level to the
selecting section 55 based on a direction signal D of the angle difference
change amount signal DV (=d .beta.fr/dt) fed from the angle difference
change amount calculating section 39 and the direction signal S of the
steering torque signal T when the direction signal D and the direction
signal S coincide with each other (signs are same), and supplies the
determining signal H03 at, for example, L level to the selecting section
55 when the direction signal D and the direction signal S differ from each
other (signs are different).
The direction signal D of the angle difference change amount signal DV is
determined to be negative (-) when the angle difference signal .beta.fr is
negative (-) and the absolute value .vertline..beta.fr.vertline. is
increasing and is determined to be positive (+) when the absolute value
.vertline..beta.fr.vertline. is decreasing.
The direction signal D of the angle difference change amount signal DV is
determined to be positive (+) when the angle difference signal .beta.fr is
negative (-) and the absolute value .vertline..beta.fr.vertline. is
increasing and is determined to be negative (-) when the absolute value
.vertline..beta.fr.vertline. is decreasing.
When the direction signal D of the angle difference change amount signal DV
(=d .beta.fr/dt) and the direction signal S of the steering torque signal
T do not coincide with each other (signs are different), the selecting
section 55 selects the multiplying section 36 (designated by bold line)
and supplies the countersteering correction amount DC or the oversteering
correction amount DO to the multiplying section 36.
Meanwhile, when the direction signal D of the angle difference change
amount signal DV (=d .beta.fr/dt) and the direction signal S of the
steering torque signal T coincide with each other (signs are same), the
selecting section 55 selects the adding section 56 (designated by broken
line) and supplies the countersteering correction amount DC or the
oversteering correction amount DO to the adding section 56.
The countersteering correction amount DC supplied to the adding section 56
is multiplied at the multiplying section 35 by the vehicle speed
coefficient KR and the angle difference change coefficient KV and a
countersteering correction amount signal IDC (=KR*KV*DC) which is a
subtracting correction signal is supplied to the subtracting section 37.
The countersteering correction amount DC or the oversteering correction
amount DO fed to the multiplying section 36 is multiplied by the vehicle
speed coefficient KR and the angle difference change coefficient KV and
the countersteering correction amount signal IDC (=KR*KV*DC) or the
oversteering correction amount signal IDO (=KR*KV*DO) is supplied to the
adding section 38 which is adding section.
As described above, when the direction signal D of the angle difference
change amount signal DV (=d .beta.fr/dt) and the direction signal S of the
steering torque signal T coincide with each other (signs are same) and the
oversteering state is directed in a converging direction, further
countersteering is not needed. Thus, the countersteering correction amount
signal IDC (=KR*KV*DC) or the oversteering correction amount signal IDO
(=KR*KV*DO) is subtracted from the target torque signal IMO and a target
torque signal IMH (=IMO-IDC, IMO-IDO) is outputted, whereby a large
reaction force is transmitted to the driver via the steering wheel.
When the direction signal D of the angle difference change amount signal DV
(=d .beta.fr/dt) and the direction signal S of the steering torque signal
T do not coincide with each other (signs are different) and the
oversteering state is directed in a diverging direction, the
countersteering correction amount signal IDC (=KR*KV*DC) or the
oversteering correction amount signal IDO (=KR*KV*DO) is added to the
target torque signal IMO and a target torque signal IMH (=IMO+IDC,
IMO+IDO) is outputted, whereby a small reaction force is transmitted to
the driver via the steering wheel, urging countersteering.
Explanation as to the understeering state and the oversteering state will
be omitted since contents thereof are the same as those explained in
relation to FIG. 3.
Although the described embodiment is arranged such that the understeering
correction amount signal DA and the countersteering correction amount
signal IDC are added by using the adding section 56 and the vehicle speed
coefficient KR and the angle difference change coefficient KV are
multiplied by the multiplying section 36, the countersteering correction
amount signal IDC (=KR*KV*DC) may be formed by providing a multiplying
section for multiplying the countersteering correction amount signal IDC
by the vehicle speed coefficient KR and the angle difference change
coefficient KV, in place of the adding section 56. The target torque
signal IMH (=IMO-IDC) may be outputted by providing a subtracting section
for subtraction-correcting the target torque signal IMO with the
countersteering correction amount signal IDC.
Being thus provided with the first direction determining section 32 for
determining coincidence or noncoincidence of the direction signal P of the
angle difference signal .beta.fr and the direction signal N of the yaw
rate signal Y, the second direction determining section 52 for determining
coincidence or noncoincidence of the direction signal P of the angle
difference signal .beta.fr and the direction signal S of the steering
torque signal T, and the selecting sections 31 and 51 for selecting the
understeering correction amount outputting section 33 when both
determination results of the first direction determining section 32 and
the second direction determining section 52 indicate that the directions
do not coincide with each other and for selecting the countersteering
correction amount outputting section 53 when a result of determination of
the first direction determining section 32 indicates that the directions
do not coincide with each other and a result of determination of the
second direction determining section 52 indicates that the directions
coincide with each other, by determining the directions of the angle
difference signal .beta.fr, the yaw rate signal Y and the steering torque
signal T, the correcting section 50 is capable of determining whether the
behavior of the vehicle is in the understeering state or excessive
countersteering state and of outputting the correction amount DA or DC in
correspondence with the understeering state or excessive countersteering
state.
Further, the correction section 50 is provided with the third direction
determining section for determining coincidence or noncoincidence of the
direction signal D of the differentiated value DV of the angle difference
signal .beta.fr and the direction signal S of the steering torque signal T
and the addition-subtraction correction sections 37 and 38 for
addition-correcting the target torque signal IMO with the adding
correction signal (oversteering correction amount signal IDO or
countersteering correction amount signal IDC) corresponding to the
oversteering correction amount DO or the countersteering correction amount
DC from the oversteering correction amount outputting section 34 or the
countersteering correction amount outputting section 53 when a result of
determination of the third direction determining section 54 indicates that
the directions do not coincide with each other, and for
subtraction-correcting the target torque signal IMO with the subtracting
correction signal (oversteering correction amount signal IDO or the
countersteering correction amount signal IDC) corresponding to the
oversteering correction amount DO or the countersteering correction amount
DC from the oversteering correction amount outputting section 34 or the
countersteering correction amount outputting section 53 when the result of
determination indicates that the directions coincide with each other. By
determining the direction signal D of the differentiated value DV of the
angle difference signal .beta.fr and the direction signal S of the
steering torque signal T, the correction section 50 can tell the driver
through a reaction force that a counter-steering operation is excessively
large or excessively small. This is achieved by adding the oversteering
correction amount DO or the countersteering correction amount DC to the
target torque signal IMO or subtracting the oversteering correction amount
DO or the countersteering correction amount DC from the target torque
signal IMO.
Further, since the correcting section 50 is provided with the angle
difference change amount calculating section 39 for calculating the change
amount DV of the angle difference signal .beta.fr and the angle difference
change coefficient generating section 40 for outputting the angle
difference change coefficient KV corresponding to the angle difference
change signal DV from the angle difference change amount calculating
section 39 and corrects the understeering correction amount DA, the
oversteering correction amount DO and the countersteering correction
amount DC by the angle difference change coefficient KV, even when the
behavior of the vehicle in the understeering state or the oversteering
state is rapidly changed, the rapid change can be transmitted quicly to a
driver as a change in the reaction force via the steering wheel.
Referring to the flowchart of FIG. 5, operation of the correcting section
shown in FIG. 4 will be discussed.
At step S1, the first direction determining section 32 determines
coincidence or noncoincidence of the direction P of the angle difference
signal and the direction N of the yaw rate signal. In the case of
coincidence, the operation proceeds to step S2 where the selecting section
31 selects the oversteering correction amount DO. Thereafter, the
operation proceeds to step S6. In the case of noncoincidence at step S1,
the operation proceeds to step S3 where the second direction determining
section 52 determines coincidence or noncoincidence of the direction P of
the angle difference signal and the direction S of the steering torque.
When the directions coincide with each other at step S3, the operation
proceeds to step S4 where the selecting section 51 selects the
countersteering correction amount DC, whereafter the operation proceeds to
step S6.
Meanwhile, when the directions do not coincide with each other at step S3,
the operation proceeds to step S5 where the selecting section 51 selects
the understeering correction amount DA, whereafter the operation proceeds
to step S9.
At step S6, the third direction determining section 54 determines
coincidence or noncoincidence of the direction D of the differentiated
value of the angle difference signal and the direction S of the steering
torque signal. In the case of noncoincidence, the operation proceeds to
step S7 by selection by the selecting section 55. In the case of
coincidence at step S6, the operation proceeds to step S9 by selection of
the selecting section 55.
At step S7, the oversteering correction amount DO or the countersteering
correction amount DC is multiplied by the vehicle coefficient KR and the
angle difference change coefficient KV by which the correction amount IDO
or the correction amount IDC is generated. Thereafter the operation
proceed to step SS where the correction amount IDO or the correction
amount IDC is added to the target torque signal IMO for correction of the
latter.
At step S9, the understeering correction amount DA, the oversteering
correction amount DO or the counter-steering correction amount DC is
multiplied by the vehicle coefficient KR and the angle difference change
coefficient KV by which the correction amount IDA, the correction amount
IDO or the correction amount IDC is generated. Thereafter, the operation
proceed to step S10 where the correction amount IDA, the correction amount
IDO or the correction amount IDC is subtracted from the target torque
signal IMO for correction of the latter.
Reference is now made to FIG. 6 which is a Lissajous diagram showing an
actual steering angle .theta. versus a difference .beta.fr between slip
angles of front and rear wheels of the vehicle carrying the electric power
steering apparatus having the correcting section of FIG. 4.
In FIG. 6, the coordinate of the actual steering angle .theta. is
represented such that the steering angle .theta. is directed in the
clockwise direction (+) in the right direction of the figure and the
coordinate of the difference .beta.fr between the slip angles of the front
and rear wheels is minus (-) when the slip angle f of the front wheel is
larger than the slip angle r of the rear wheel.
A running vehicle is brought into a linearly progressing state when both
the actual steering angle .theta. and the difference .beta.fr between the
slip angles of the front and rear wheels are .theta. at an intersection of
the coordinate of the actual steering angle .theta. and the coordinate of
the difference .beta.fr between the slip angles of the front and rear
wheels.
In accordance with an increase in the actual steering angle .theta. in the
clockwise direction (+ direction) from the state, the difference .beta.fr
between the slip angles of the front and rear wheels increases in the
minus (-) direction and the vehicle keeps the linearly progressing state.
In the state (fourth quadrant of coordinate), the slip angle f of the front
angle tends to increase more than, the slip angle .beta.r of the rear
wheel in respect of the increase in the actual steering angle .theta. and
the understeering region where slippage of the front wheel is large is
formed.
Further, a portion of the understeering region where the difference
.beta.fr between the slip angles of the front and rear wheels is disposed
at a vicinity of 0, is referred to as a weak understeering region and a
portion thereof in a range where the difference .beta.fr between the slip
angles of the front and rear wheels is large is referred to as an
excessively large understeering region.
Further, a state where the difference .beta.fr between the slip angles of
the front and rear wheels is disposed at a vicinity of 0 even when the
steering angle .theta. is increased from a state where both the actual
steering angle .theta. and the differences .beta.fr between the slip
angles of the front and rear wheels are 0, is referred to as a neutral
steering region.
When the difference .beta.fr between the slip angles of the front and rear
wheels is shifted to plus (+) from the neutral steering region (state
where the slip angle .beta.r of the rear wheels is larger than the slip
angle .beta.f of the front wheels and the rear wheels is slipping), the
vehicle enters the overateering region (first quadrant of coordinate) and
the vehicle spins when the oversteering state continues and the slip angle
.beta.r of the rear wheel is increased.
To prevent the vehicle from spinning, the steering wheel is operated in the
counterclockwise direction, the actual steering angle is increased in the
counterclockwise direction (- direction), slippage of the rear wheel is
restrained and the slip angle .beta.r of the rear wheel is controlled to
reduce by which the vehicle can be recovered to the linearly progressing
state at the second quadrant of coordinate.
The region (second quadrant of the coordinate) is referred to as
countersteering region.
However, when the countersteering becomes excessive, the vehicle is
deviated from a linearly progressing line and becomes remote from the
radius.
When an amount of initial countersteering is made large, in order to make
the vehicle ride on the linearly progressing line, a number of times of
carrying operation in the clockwise direction and the counterclockwise
direction of the steering wheel must be repeated and the behavior of the
vehicle becomes irregular.
By applying the correcting section 50 discussed in relation to FIG. 4, the
understeering state, oversteering state and excessive countersteering
state of the vehicle are transmitted to the driver as a road reaction
force via the steering wheel by which the driver can significantly improve
control of the behavior of the vehicle (drifting run) explained in
reference to FIG. 6.
FIG. 7 is a schematic view illustrating a drift run of the vehicle carrying
the inventive electric power steering apparatus.
In relation to FIG. 7, explanation will be given of the operation wherein a
progressing line of the vehicle is changed by a drift run at a curve
having a radius of R.
When the steering wheel is operated in the right (clockwise) direction from
a state (1) where the vehicle is progressing linearly, the slip angle
.beta.f of the front wheel becomes larger than the slip angle .beta.r of
the rear wheel, whereby the understeering state (2 and 2) is produced.
As the steering wheel is further operated in the right (clockwise)
direction, the slip angle .beta.r of the rear wheel becomes larger than
the slip angle .beta.f of the front wheel, whereby the vehicle enters the
oversteering state (4).
When the slip angle .beta.r of the rear wheel becomes far larger than the
slip angle .beta.f of the front wheel by continuing the oversteering
state, the vehicle is spun. Thus, the assist torque is reduced and a large
reaction force is transmitted to the driver.
The driver becomes conscious of a small assist torque and a large reaction
force via the steering wheel and operates the steering wheel to the left
(counterclockwise) direction. Then, the vehicle is brought into the
countersteering state (5), the assist torque is reduced, and a large road
reaction force is transmitted to the driver.
In this moment, when the driver carries out the steering operation in the
counterclockwise direction more than necessary, the slip angle .beta.f of
the front wheel becomes larger than the slip angle .beta.r of the rear
wheel. Since the direction of the steering torque is in the
counterclockwise direction, the excessive countersteering state where the
assist torque is large and the road reaction force is small is produced.
When the steering wheel operation is continued as it is, the vehicle is
progressed from the corner toward an outer side.
When the driver operates steering wheel in the clockwise direction to avoid
the situation, the slip angle .beta.r of the rear wheel becomes larger
than the slip angle .beta.f of the front wheel and the wheel is brought
from the excessive countersteering state again to the oversteering state.
In this way, when the excessive countersteering state is produced from the
start, the recovery operation is difficult and the behavior of the vehicle
(drifting run) becomes unstable.
Accordingly, the steering wheel is operated (7 through 8) such that the
slip angle .beta.f of the front wheels comes gradually nearer to the slip
angle .beta.r of the rear wheel and is made finally to become equal
thereto in the countersteering operation by which the vehicle is brought
into a linear progressing state (9) and the corner is cleared.
As thus far explained, according to the electric power steering apparatus
of the present invention, the control unit is provided with the slip angle
difference predicting section for predicting the difference between the
slip angle of the front wheel and the slip angle of the rear wheel and the
correcting section for correcting the target torque signal based on the
angle difference signal from the slip angle difference predicting section
in which the behavior of the vehicle is predicted from the angle
difference signal, the target torque signal is corrected by the correction
amount in correspondence with the angle difference signal, the motor for
adding the assist torque to the steering system is controlled to drive in
consideration of influence of a change in road reaction force and
accordingly, a driver is given agile steering feeling in normally running
the vehicle and in the understeering region or the oversteering region
where the behavior of the vehicle is unstable, the driver can carry out
optimum steering wheel operation in respect of the behavior of the vehicle
by transmitting road reaction force to the driver.
Further, the slip angle difference predicting section according to the
present invention calculates the angle difference based on the turn angle
signal detected by the turn angle sensor for detecting the turn angle of
the front wheel, the vehicle speed signal detected by the vehicle speed
sensor, the yaw rate signal detected by the yaw rate sensor and the
dimensional parameter of the vehicle and can calculate the angle
difference by using the existing sensors mounted on the vehicle without
actually detecting the angle difference and the differentiating circuit is
not included in the calculation and accordingly, noise is not mixed and
the accurate slip angle difference can be predicted.
Further, the correcting section according to the present invention
determines whether the behavior of the vehicle is in the understeering
region or the oversteering region depending on the direction detected by
the determining signal and carries out correction in correspondence with
the behavior of the vehicle and accordingly, the driver can accurately
recognize the behavior of the vehicle in accordance with whether the
vehicle is in the understeering state or the oversteering state.
Further, the correcting section according to the present invention can
transmit large reaction force by reducing assist torque by correcting to
subtract an amount from the target torque signal in the understeering
region and can transmit small reaction force by correcting to subtract an
amount from the target torque signal in the oversteering region and the
characteristics can be set separately and accordingly, the driver can be
conscious of the behavior of the vehicle and can carry out optimum
steering wheel operation in correspondence with the behavior of the
vehicle by intention of the driver.
Further, the correcting section according to the present invention can
correct the understeering correction amount and the oversteering
correction amount by the angle difference change coefficient and
accordingly, can realize precise steering with fast response also in
respect of rapid change in the behavior of the vehicle.
Further, the correcting section according to the present invention can
determine whether the behavior of the vehicle is in the understeering
state or the excessive countersteering state by determining the directions
of the angle difference signal, the yaw rate signal and the steering
torque signal and can output the correction amount in correspondence with
the understeering state, oversteering state or the excessive large
countersteering state and accordingly, a capacity of the driver can
optimally be achieved even in the critical region of the behavior of the
vehicle as in drifting run.
Further, the correcting section according to the present invention can
transmit whether an amount of operating countersteering is excessively
large or excessively small to a driver by reaction force by adding the
oversteering correction amount or the countersteering correction amount to
the target torque signal or subtracting it from the target torque signal
by determining the direction of the differentiated value of the angle
difference signal and the direction of the steering torque signal and
accordingly, the capacity of the driver can be achieved to a limit even in
the critical region of the behavior of the vehicle as in drifting run.
Further, the correcting section according to the present invention can
correct the understeering correction amount, the oversteering correction
amount and the countersteering correction amount by the angle difference
change coefficient and accordingly, even when the behavior of the vehicle
in the understeering, oversteering or countersteering state is rapidly
changed, the rapid change can be transmitted to the driver swiftly as a
change in the reaction force via the steering wheel and accordingly, the
driver can swiftly deal therewith even in the critical region of the
behavior of the vehicle as in drifting run or the like.
Accordingly, there is provided an electric power steering apparatus in
which even when lateral acceleration G with friction coefficient as a
parameter is in a nonlinear region or in the critical region of the
vehicle behavior of understeering, oversteering and excessive
countersteering state, the driver can accurately be conscious thereof and
can carry out optimum steering desired by the driver without setting the
reference lateral acceleration based on the friction coefficient of a road
which is difficult to detect.
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